Coatings that provide low reflectivity or a high percent transmission over a broad wavelength range of light are desirable in many applications including semiconductor device manufacturing, solar cell manufacturing, glass manufacturing, and energy cell manufacturing. The refractive index of a material is a measure of the speed of light in the material which is generally expressed as a ratio of the speed of light in vacuum relative to that in the material. Single layer low reflectivity coatings generally have a refractive index (n) in between air (n=1) and glass (n˜1.5).
An anti-reflective (AR) coating is a type of low reflectivity coating applied to the surface of a transparent article to reduce reflectivity of visible light from the article and enhance the transmission of such light into or through the article. One method for decreasing the refractive index and enhancing the transmission of light through an AR coating is to increase the porosity of the anti-reflective coating. Porosity is a measure of the void spaces in a material. Although such anti-reflective coatings have been generally effective in providing reduced reflectivity over the visible spectrum, the coatings have suffered from deficiencies when used in certain applications. For example, porous metal oxide AR coatings which are used in solar applications are highly susceptible to moisture absorption due to their affinity for water (hydrophilicity). Moisture absorption may lead to an increase in the refractive index of the AR coating and corresponding reduction in light transmission.
Magnesium fluoride thin films can be deposited by evaporation resulting in columnar and dense films, which can be unsuitable for anti-reflective coatings, Sol-gel methods can produce magnesium fluoride thin films using colloidal crystalline MgF2 nanoparticles, which can be sintered at high temperatures. Magnesium fluoride thin films can also be formed by exposing magnesium oxide to fluorine-containing vapors. These processes to form magnesium fluoride thin films can provide minimum control over the porosity level of the coated layers, resulting in limited ranges of index of refraction. However, magnesium fluoride is hard to deposit by sputtering methods.
Large scale glass coating (>1 m2) requires sputtering methods for low cost and uniform deposition. Thus, there is a need for AR coatings which can be easily sputtered with high deposition rate and low refractive index with no absorbing materials.